1. Field of the Invention
The present invention relates to orthopedic implants of the type having a porous surface into which bone tissue can grow or bone cement can enter and, more particularly, to a method of bonding a porous metal structure, such as porous titanium or porous tantalum onto a metal substrate preferably comprising a titanium-based or cobalt-based alloy.
2. Description of the Related Art
Orthopedic implant devices commonly include a porous structure of desired thickness, generally 0.5 to 5.0 mm, on the bone contacting surface of the implant to promote bone growth there through and to enhance attachment of the device to adjacent bone tissue. Growth of bone into an implant is advantageous in that the same allows for increased fixation of the implant.
Accordingly, it is desirable to promote as much bone growth into an implant as possible. Various methods have been developed for manufacturing an orthopaedic implant device having a porous surface, including plasma spraying of metal powder, sintering of metal beads, and diffusion bonding of metal wire mesh. See for example, the following patents, the disclosures of which are hereby incorporated by reference and briefly described herein.
U.S. Pat. No. 3,906,550 to Rostoker et al. discloses a porous metal structure adapted for attachment to a prosthesis. The fiber metal is molded into the desired shape using dies. The fiber metal is then sintered together to form metallurgical bonds within the pad and between fiber metal pad and the substrate.
U.S. Pat. No. 3,605,123 to Hahn discloses a metallic bone implant having a porous metallic surface layer. The porous layer may be secured to the implant by a plasma spray method or by other suitable means.
U.S. Pat. No. 4,636,219 to Pratt et al. discloses a prosthesis including a porous surface comprised of a layered metal mesh structure and a process for fabricating the mesh screen structure for bonding to the prosthesis. The mesh may be bonded to a thin substrate which can then be cut or formed and applied to the body of a prosthesis on a flat surface or contoured into specific shapes by forming.
U.S. Pat. No. 4,570,271 to Sump discloses a prosthesis with a porous coating in which the porous coating is preformed directly into the desired shape which corresponds to the preselected surface of the prosthesis. The preformed porous coating is then overlaid onto the preselected surface, compressed, and heated to adhere the preformed porous coating to the prosthesis.
U.S. Pat. No. 3,855,638 to Pilliar described the bonding process to a prosthetic device having a solid metallic substrate with a porous coating adhered thereto. A slurry of metallic particles was applied to the substrate, dried and then sintered to establish metallurgical bond between particles and the substrate.
U.S. Pat. Nos. 5,198,308 and 5,323954 entitled “Titanium Porous Surface Bonded to a Cobalt-Based Alloy Substrate in Orthopaedic Implant Device and Method of Bonding Titanium to a Cobalt-Based Alloy Substrate in an Orthopaedic Implant Device” which are assigned to assignee of the present invention teaches diffusion bonding of titanium fiber metal pad porous layer to Co—Cr—Mo alloy implants with the use of a thin titanium and or L-605 alloy foil to increase the bond strength of the coating to the substrate and corrosion resistance of the implant.
U.S. Pat. No. 5,104,410 granted to Chowdhary discloses the method of making a surgical prosthetic device, comprising of a composite structure having a solid metal substrate and a porous coating with multiple sintered layers. The porous coating has an external layer to accept bone ingrowth and the chemical composition of the external layer is same as the intermediate layer between the porous coating and the implant surface. The intermediate layer bonds the external porous layer to the substrate. These layers are applied in a process of multiple sintering where each successive layer is individually sintered to the substrate or the proceeding layer, as applicable. This process provides a porous layer having increased strength of attachment between the substrate and the external porous layer.
Titanium is a known biocompatible metal that is often used in orthopedic applications. Porous titanium or porous titanium alloy can be used on the bone contacting surface of an orthopedic implant to promote bone growth there through. Tantalum is another known biomaterial. Tantalum is known to be particularly adept at promoting bone growth. Implex, Inc. markets a structured porous tantalum metal biomaterial, described in U.S. Pat. No. 5,282,861, for orthopedic use under the trade name HEDROCEL®. HEDROCEL is described as being more than 80% porous, and closely resembles human trabecular bone in both physical and mechanical properties. In spite of the value of using a porous layer in orthopedic implants, bonding porous metal to a metal substrate such as cobalt alloy or titanium alloy has been difficult, especially in the case of HEDROCEL. The reason for this difficulty is that metallurgically bonding two components generally requires a large amount of contact between the surfaces at which the bond is desired. The porosity of HEDROCEL results in sparse contact with an opposing metal substrate, thereby making sintering or diffusion bonding difficult. Moreover, this porosity also makes it difficult to maintain the narrow dimensioning tolerances for machined HEDROCEL, components. The binding mixture, therefore, also serves to fill in “gaps” or “spaces” that may exist between a HEDROCEL porous layer of desired shape and a corresponding metal substrate.
Thus, a need exists for a method of bonding a porous metal structure to a metal substrate.
An additional need exists for a method of bonding a porous metal surface to a component of an orthopedic implant device comprising a solid metal, such as cobalt-chrome alloy or titanium alloy.